The film is set before, during and after the Great War in several different parts of France, Austria, and Germany. Jules (Oskar Werner) is a shy writer from Austria who forges a friendship with the more extroverted Jim (Henri Serre). They share an interest in the world of the arts and the Bohemian lifestyle. At a slide show early in the movie, they become entranced with a statue of a goddess and its serene smile.
After encounters with several women, they meet the free-spirited, capricious Catherine (Jeanne Moreau), a doppelgänger for the statue with the serene smile. Although she begins a relationship with Jules, both men are affected by her presence and her attitude toward life. A few days before the declaration of war, Jules and Catherine move to Austria to get married. Both men serve during the war; however, they serve on the opposing sides, and each fears throughout the conflict that he might have killed the other.
After the wartime separation, Jim visits, and later stays with, Jules and Catherine in their house in the Black Forest. Jules and Catherine have a little daughter, Sabine, but the marriage is not a happy one. Catherine torments and punishes Jules with numerous affairs, and he tells Jim that she once left Jules and their daughter for six months. She flirts with and attempts to seduce Jim, who has never forgotten her. Jules, desperate that Catherine might leave him forever, gives his blessing for Jim to marry Catherine so that he may continue to visit them and see her. For a while, the four of them live happily together in the same chalet in Austria, until tensions between Jim and Catherine arise because of their inability to have a child. Jim leaves Catherine and returns to Paris. After several exchanges of letters between Catherine and Jim, they resolve to reunite when it is discovered that Catherine is indeed with child after all. However, the reunion does not occur after Jules writes to inform Jim that his and Catherine's unborn baby has miscarried.
After a time, Jim runs into Jules in Paris. He finds that Jules and Catherine have returned to France. Catherine attempts to win Jim back, but he rebuffs her, saying he is going to marry Gilberte. Furious, she pulls a gun on him, but he wrestles it away and flees. He later encounters Jules and Catherine in a famous (at that time) movie theater, the Studio des Ursulines.
The three of them stop at an outdoor cafe. Catherine asks Jim to get into her car, saying she has something to tell him. She asks Jules to watch them and drives the car off a broken bridge, killing herself and Jim. Jules is left to deal with the ashes of his friends.